I use:
IQueryFilter queryFilter = new QueryFilterClass();
with ArcGIS .NET SDK and build a query...

Does anybody had problems using UPPER(stringField) and LOWER(stringField) functions on esri file geodatabases? If so, please share links to some "confirmed-will-fix-in-2-years" pages...

I know we have some "old" warnings in our codebase about that. Like we though UPPER(stringField1)=UPPER(stringField2) doesn't work on file gdb. Last week I ran into it again but it seems I could not get even UPPER(stringField)=('STRING-IN-mixedCACE'.toUpper()) query to find anything in gdb.
UPPER(stringField)='STRING-IN-UPPERCACE' works, but the UPPER should be better named NOOP.

Example. There is a record in fgdb where stringField='Abc'. The query UPPER(stringField)='ABC' returns 0 records. (Error: Should be 1!)

PS. Such UPPER/LOWER queries seem to work correctly in ArcMap when using feature layer definition query.

Perhaps I am misunderstanding your question, but I am not sure what results you expect.. UPPER(stringField)='mixedCASE' will never match anything. UPPER(stringField='ABC') will never match anything as well. UPPER(stringField)='ABC' is the correct expression.
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Petr KrebsFeb 20 '12 at 19:28

Thats for you attention, you are correct. Did some edits.
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r.pankeviciusFeb 20 '12 at 19:32

I stopped at a breakpoint and used that nice Visual Studio feature immediate window or so. (I tried to q-search for an exact name but it looks other guys like other features so that name may be not exactly correct.) It looks quoted names (LAST_NAME -> "LAST_NAME") do not work either.
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r.pankeviciusFeb 27 '12 at 21:06

Then I tried to write a 'failing unit test', but (probably) found there are some issues with setting up licenses in MTA. q-searched... Hrr. There are too way many "powerpoint programmers" in esri... (btw, did you notice that lowercase?!) So this may take some time.
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r.pankeviciusFeb 27 '12 at 21:13